1. Field of the Invention
This invention relates to an image forming apparatus such as a copying machine, a facsimile machine, or a printer, and more particularly to an image forming apparatus and method for controlling a charging voltage and a developing bias voltage.
2. Discussion of the Background
A method of forming a toner image having a sharp edge by applying an AC bias voltage as a developing bias voltage when developing a latent image with toner is well known in an image forming apparatus, as disclosed, for example, in Japanese Laid-Open Patent Publication No. 6-258873.
Also, an image forming apparatus is known, in which a so-called negative-to-positive developing method is used and an AC bias voltage is applied to a developer bearing member carrying toner when developing a latent image with the toner. In such an apparatus, when an image forming operation is started in accordance with a starting instruction from a start switch, a voltage is applied to a charging device from a power source such as a power pack, and the charging device uniformly charges the surface of an image bearing member such as an optical photoconductive drum (hereinafter called an OPC drum). A latent image is formed on a charged surface of the image bearing member with an optical writing device. The toner, which is carried by the developer bearing member and which has the same polarity as that of the charging voltage, is then applied from the developer bearing member to the latent image on the image bearing member, so as to form a toner image, by applying an AC bias voltage as a developing bias voltage to the developer bearing member from the power source. The toner image is then transferred to a transfer medium such as a sheet of transfer paper or an intermediate transfer medium by a transfer device.
In the method disclosed in JP No. 6-258873, a developer bearing member is disposed opposite an image bearing member, and an oscillating bias voltage is applied to the developer bearing member. The oscillating bias voltage includes a first peak voltage for energizing developer which is carried by the developer bearing member to transfer developer to a latent image portion to be visualized, and a second peak voltage for energizing developer moved to the latent image portion back to the developer bearing member. The voltage level of the first peak voltage is between the potential of the latent image portion and the potential of a background portion of the latent image. An absolute value of the first peak voltage (V1) is more than an absolute value of the potential of the latent image portion (VL), and an absolute value of the second peak voltage (V2) is less than that of the potential of the latent image portion (VL) (i.e. V1&gt;VL&gt;V2).
In the above-described image forming apparatus using the negative-to-positive developing method, in which development is performed with toner having the same polarity as that of the charging voltage for the image bearing member, toner is transferred to the image bearing member from the developer bearing member when an absolute value of the surface potential of the image bearing member is smaller than that of the potential of the developer bearing member and thereby a toner image is formed on the image bearing member. When an absolute value of the surface potential of the non-image portion of the image bearing member, such as a charged area preceding a latent image portion of the image bearing member, is smaller than that of the potential of the developer bearing member, toner is also transferred to such a non-image portion of the image bearing member.
Therefore, in order to avoid unnecessary adhering of toner to a charged area preceding a latent image portion of the image bearing member, when a first image forming operation is performed (for example, when a first copy is made by a copying machine), a developing bias voltage is applied to the developer bearing member after a tip portion of the charged area of the image bearing member charged with the charging device reaches a developing area where the developer bearing member and the image bearing member oppose each other, such that the absolute value of the potential of the developer bearing member becomes smaller than the absolute value of the surface potential of the charged area preceding the latent image portion of the image bearing member.
Further, when an absolute value of the surface potential of the image bearing member is excessively greater than that of the potential of the developer bearing member, a problem of adhering of carrier to the image bearing member arises in a two-component developing device using a mixture of toner and carrier, and a problem that toner having an opposite polarity to that of the image bearing member adheres to the image bearing member arises in a one-component developing device using only toner. The carrier adhered to the image bearing member causes, for example, breaking of a cleaning blade and thereby damage to the image bearing member or disturbing a toner image to be formed in the subsequent image forming operation. The toner adhered to the image bearing member causes, for example, to disturb a subsequent toner image to be formed thereupon.
More specifically, for example, when the potential of a surface of an image bearing member is -880V and the developing bias voltage is not applied (the developing bias voltage is 0V), the surface potential of the image bearing member has a potential difference of 880V relative to the potential of the developing bias. This produces a strong electric field in which a positively charged carrier or toner particle is easily transferred to the image bearing member. Therefore, even when a developing bias voltage is applied, if the applied developing bias potential does not rise fast enough and the surface potential of the image bearing member is excessively greater than that of the potential of the developing bearing member, the problem of adhering of carrier and/or toner to the image bearing member occurs, such as, for example, to a charged area preceding a latent image portion of the image bearing member.
Therefore, in order to avoid such unnecessary adhering of carrier and toner having an opposite polarity to that of the image bearing member to the image bearing member, when a first image forming operation is performed, a developing bias voltage is required to be applied to the developer bearing member before a portion of the image bearing member which is fully charged to a predetermined level reaches a developing area where the developer bearing member and the image bearing member oppose each other.
In the image forming apparatus using the negative-to-positive developing method, therefore, a developing bias voltage is generally applied at the same time when the tip portion of a charged area of the image bearing member reaches the developing area where the developer bearing member and the image bearing member oppose each other.
However, depending on a response speed of each power pack for the charging device and the developer bearing member, a developing bias voltage V.sub.B may rise faster than a charging voltage for the image bearing member as illustrated for example in FIG. 8. This may result in the developing bias voltage V.sub.B exceeding the charged potential V.sub.D at a tip portion of a charged area of the image bearing member, which is not charged to a fully charged level 10 when charged by the charging device and which precedes a latent image portion of the image bearing member, and thereby toner adheres to the tip portion of the charged area of the image bearing member preceding the latent image portion, depending on a potential difference between the developing bias voltage V.sub.B and the charged potential V.sub.D of the image bearing member. The toner adhered on the image bearing member is transferred to a contact transfer device in contact with the image bearing member, and stains the contact transfer device. The stained contact transfer device subsequently stains the surface of the image bearing member, which results in disturbing subsequent toner images to be formed.
If the response speed of the power pack for the charging device is sufficiently fast as the power pack for the developer bearing member and the image bearing member is charged such that the absolute value of the charged potential V.sub.D of the image bearing member is greater than the absolute value of the developing bias voltage V.sub.B in the development, the problem of adhering of toner to the image bearing member does not arise. However, a power pack having a fast response speed is large and expensive.